The present invention relates to a video signal processing circuitry and, more particularly, to circuitry adapted to improve a video signal-to-noise (SN) ratio by utilizing the vertical correlation in a picture.
In the known means for improving a video signal-to-noise ratio, there is a comb filter of the type that utilizes the vertical correlation in a picture. The vertical correlation is defined here as a characteristic that any one-line component of a video signal obtained by scanning an image in a standard television system resembles the next one-line component. Meanwhile, noise components included in the video signal have no positive correlation between lines unlike the above-mentioned video components and are mutually uncorrelated in general. The principle of a comb filter resides in utilizing the interline correlation of video signal components and the noncorrelation of noise components. In its simplest constitution, there is employed a delay line whose delay time corresponds to one horizontal scanning repetition period, and input and output video signals thereof are added to each other. The absolute value .vertline.G .vertline. of the gain in the transfer characteristic of such comb filter is given by EQU .vertline.G .vertline.=.vertline.cos (.pi.f/fH).vertline. (1)
where fH is a horizontal scanning frequency. The term "comb filter" is derived from its characteristic that a peak point and a dip point obtained when f=nfH (n=0, 1, 2, . . . ) and f=(n+1/2) fH respectively are equispaced like the teeth of a comb. As known well, the frequency spectrum of a video signal having a close interline correlation concentrates on frequencies near nfH, so that the comb filter of the above-mentioned characteristic causes little loss to the video signal while suppressing merely the noise components existent near (n+1/2) fH to consequently improve the SN ratio.
In the prior art developed for reducing the response at frequencies near (n+1/2) fH in a wider range to further enhance the effect of improving the SN ratio, one example is disclosed in "NEW SYSTEMS FOR VIDEO PROCESSING ICs" by Noriyuki Yamashita et al. IEEE transactions on Consumer Electronics, pp. 179-194, vol. CE-29, No. 3, August 1983. The circuitry described therein includes a feedback circuit with a variable attenuator to render the comb characteristic by selectively setting a feedback coefficient K. However, one of the technical difficulties existing in such circuitry is concerned with a phenomenon that the gain at a frequency fnH varies in accordance with change of the coefficient K to eventually cause a variation in the signal output level. And no adequate contrivance has been proposed heretofore to eliminate blur induced in the vertical contour of a picture concomitant with the use of such a comb filter.